1. Field of the Invention
The present invention relates to a stepping motor structure, a stepping motor device and a driving method thereof to rotate a rotary polygon mirror for measuring the inter-vehicle distance, direction and relative speed of a preceding vehicle.
2. Description of Related Art
Conventionally, a stepping motor using a permanent magnet for a rotor has been often used for driving a rotary portion of an office machine such as a printer, high-speed facsimile equipment, a copying machine for PPC (Plain Paper Copier) or the like, since the stepping motor has a high efficiency. Moreover, there has been a recently growing interest in preventive safety technologies to prevent traffic accidents from occurring by making vehicles more intelligent. One such preventive safety technologies to be realized is driving environment recognition, specifically inter-vehicle distance control technology using laser radar and image recognition technology. A device for such a technology requires a laser scanner using a stepping motor and a rotary polygon mirror.
A two-phase stepping motor is mainly employed for use requiring a medium accuracy, while a three-phase stepping motor excellent in cost performance is employed for use requiring high-accuracy, low vibration and low noise. As a stepping motor for office machines such as a laser printer and facsimile equipment or the like requiring accuracy in positioning and little unevenness in rotation, a three-phase machine has been often employed in order to obtain high resolution and high torque. The three-phase machine comprises a cylindrical permanent magnet type rotor formed with multiple magnets in a cylindrical shape, or a hybrid type rotor having a permanent magnet held between two magnetic plates formed with multiple pole teeth, and a stator formed with pole teeth opposite the rotor surface.
The stepping motor having the permanent magnet rotor was accurately step-driven one step angle at a time by a driving pulse input from an outer part and an output shaft of the motor rotated as if intermittently driven. Moreover, there was a growing tendency to use rare earth magnets for the permanent magnet to be employed as a rotor in order to obtain higher driving torque. The permanent magnet is magnetized with different poles alternately in strips circumferentially and the magnet surface magnetic flux density measured along the circumference of the magnet was shown as a substantially trapezoidal distribution. Further, a trapezoidal or triangular surface of the magnetic pole teeth of a stator yoke is often employed in order to obtain high torque.
The three-phase machine provided with the cylindrical permanent magnet type rotor or the hybrid type rotor and the stator formed with pole teeth is capable of obtaining high resolution and high torque as mentioned above. Since the distribution of the surface magnetic flux density of the magnetized permanent magnet is substantially trapezoidal circumferentially, step-like driving is easily achieved when the output torque is increased. On the other hand, it also has drawbacks such as increased vibration upon driving and stopping the rotor and it makes smooth driving difficult. That is, noise or vibration is generated by a vibration torque component contained in the torque generated by the product of excitation electric currents and field magnetic flux density. Accordingly, the above-mentioned construction wherein a large number of harmonics are contained in the field magnetic flux density generated in an air gap between the permanent magnet of the rotor and the stator increases noise and vibration.
Publications of Japanese Patent Application Nos. Hei 05-221388 and Hei 09-325197 disclose a permanent magnet type stepping motor to lower damping of a rotor in starting or stopping a motor by solving such problems and rotating the rotor smoothly.
According to the publication of Japanese Patent Application No. Hei 05-221388, the permanent magnet of the rotor is skew-magnetized, the magnetic flux distribution in the magnetized surface of the permanent magnet is made to be substantially sinusoidal circumferentially and the magnetic pole teeth of a stator yoke are made to be rectangular.
Moreover, according to the publication of Japanese Patent Application No. Hei 09-325197, the relation between the number of stator poles Q and the number of S-pole and N-pole pairs of the rotor N is Q=6 k, and are set so as to satisfy N=yk(6nxc2x11), and the magnetic poles of the stator are excited in a two-phase/three-phase excitation mode.
The stepping motor in accordance with each of the publications as mentioned above, however, has the following problems. Although the magnetized state of the rotor in a substantially sinusoidal shape circumferentially is to lower vibration and noise in Japanese Patent Application No. Hei 05-221388, the permanent magnet has an outer diameter configuration in which a slightly uneven cross-section which continues in each of the magnetic poles and is furthermore skew-magnetized, which makes manufacturing difficult. Moreover, the publication only discloses that a suitable number of magnetic poles (N-pole, S-pole) are skew-magnetized, but it does not describe the number of poles.
According to the publication Patent Application No. Hei 05-221388, the relation between the number of the stator poles and the number of pairs of rotors is set so that the magnetic poles of the stator are excited in a two-phase/three-phase excitation mode; however, the magnetized state of the rotor is not described.
Further, when there is a steep load change of a rotating shaft and the rotation stops due to step out, the stepping motor according to each of the above-mentioned publications remains in the stopped state. As a result, excessive current flows when the resistance value of stator windings is small without back electromotive force being generated. The flow of the excessive current increases the temperature of an adhesive or the like fixing the windings, which may cause problems such as poor insulation.
In view of the foregoing problems, it is an object of the present invention to provide a stepping motor having low vibration, a stepping motor device which prevents the stepping motor from being damaged by restarting in a case of a steep load change of a rotating shaft and the rotation stops due to step out, and a driving method thereof.
In order to achieve the foregoing object, a stepping motor according to a first aspect of the present invention comprises a permanent magnet type rotor with a plurality of poles secured to a rotating shaft and a stator having stator magnetic poles with stator magnetic pole teeth in which excitation windings are wound on a plurality of magnetic poles in a star or delta connection, wherein the rotor is magnetized in different directions alternately circumferentially to satisfy the following equation: M=4F/3 where M is the number of poles of the rotor and F is the number of the stator magnetic poles, the rotor is cylindrical in shape with the stator rotatably disposed inside, disposed opposing the surfaces of the stator magnetic pole teeth through an air gap which is of a uniform dimension throughout the circumference between the surfaces of the stator magnetic pole teeth of the stator and the rotor, and the surface magnetic flux distribution thereof has a substantially sinusoidal wave form circumferentially. This results in a stepping motor with smooth rotation. Also, in a conventional stepping motor, in order to make the magnetic flux distribution after magnetization approach a rectangular shape, a magnetic body such as iron is further provided on the magnetic body that has been magnetized to make the magnetic flux uniform. According to the present invention, however, a permanent magnet type rotor construction can be simplified by having the magnetic flux magnetized in a substantially sinusoidal shape circumferentially without providing a magnetic body such as iron for making the magnetic flux uniform.
In a preferred form according to the first aspect of the present invention, in the stepping motor, a cylindrical bearing holder is provided for securing the rotating shaft in a predetermined location in an enclosure, the bearing holder vertically mounted by caulking to a base on which the stepping motor is mounted, and the rotor is disposed opposing the outside of the stator through the air gap which is of a uniform dimension throughout the circumference between the rotor and the stator magnetic pole teeth surfaces and secured to the rotating shaft rotatably provided by a pair of bearings opposing one another through the bearing holder for securing the rotating shaft. As a result, a stepping can be mounted to the base using a simple construction.
In another preferred form according to the first aspect of the present invention, in the stepping motor, the bearing holder has an arc-shaped deformation preventing groove to prevent deformation due to the caulking. Accordingly, it can be prevented to occur mounting problems such as the stepping motor detaching from the base after it has been mounted thereon.
In still another preferred form according to the first aspect of the present invention, in the stepping motor, the arc-shaped deformation preventing groove is provided along the circumference at the side end contacting the base to which the stepping motor is mounted. Accordingly, occurrence of mounting problems such as the stepping motor detaching from the base due to external force from various directions after the motor has been mounted on the base can be prevented.
In yet another preferred form according to the first aspect of the present invention, in the stepping motor, the rotor is provided opposing the stator magnetic poles on a rotor yoke secured to the rotating shaft and a notch is provided in the rotor yoke in order to leak magnetism of the rotor, and a leakage flux detector for detecting leaking magnetic flux from the rotor is provided in a position opposing the notch. As a result, the stepping motor can detect stopping of the stepping motor due to a steep load change.
In another preferred form according to the first aspect of the present invention, in the stepping motor, a leakage flux detector for detecting changes in magnetic poles is provided on a cylinder end surface of a cylindrical permanent magnet provided in a cylindrical shape opposing the stator magnetic poles on the rotor yoke secured to the rotating shaft. As a result, the stepping motor can detect the positions of magnetic poles of the permanent magnet.
In yet another preferred form according to the first aspect of the present invention, in the stepping motor, a rotary polygon mirror secured to the rotating shaft which is rotatably provided through the cylindrical bearing holder vertically mounted on the base to which the stepping motor is mounted, which rotates along with the rotating shaft, is provided on the outer periphery of the rotor yoke with each mirror surface corresponding to a magnetic pole of the permanent magnet rotor of the stepping motor. As a result, a device for measuring the inter-vehicle distance, direction, and relative speed of a preceding vehicle can be made less expensive.
A stepping motor device according to a second aspect of the present invention comprises: a stepping motor including a permanent magnet type rotor with a plurality of poles secured to a rotating shaft, a stator having stator magnetic poles with stator magnetic pole teeth in which excitation windings are wound on a plurality of magnetic poles in a star or delta connection, and a rotary polygon mirror provided on the outer periphery of a rotor yoke rotatable along with the rotating shaft with each mirror surface corresponding to a magnetic pole of the rotor, wherein the rotor is magnetized in different directions alternately circumferentially to satisfy the following equation: M=4F/3 where M is the number of poles of the rotor and F is the number of the stator magnetic poles, the rotor is cylindrical in shape with the stator rotatably disposed inside, disposed opposing to the surfaces of the stator magnetic pole teeth through an air gap which is of a uniform dimension throughout the circumference between the surfaces of the stator magnetic pole teeth of the stator and the rotor, and the surface magnetic flux distribution thereof has a substantially sinusoidal wave form circumferentially; a leakage flux detector for detecting changes in magnetic poles provided on a cylinder end surface of the rotor of the stepping motor; a driving means to control rotation of the stepping motor by impressing a driving signal in a three-phase single-two-phase excitation mode to three excitation feeding terminals in a star or delta connection wound on a plurality of magnetic poles of the stepping motor; and a means to detect the position of the rotary polygon mirror by a signal from the leakage flux detector. As a result, a device for measuring the inter-vehicle distance, direction, and relative speed of a preceding vehicle is can be made less expensive.
A stepping motor device according to a third aspect of the present invention comprises: a stepping motor including a permanent magnet type rotor with a plurality of poles secured to a rotating shaft, and a stator having stator magnetic poles with stator magnetic pole teeth in which excitation windings are wound around a plurality of magnetic poles in a star or delta connection, wherein the rotor is magnetized in different directions alternately circumferentially to satisfy the following equation: M=4F/3 where M is the number of poles of the rotor and F is the number of the stator magnetic poles, the rotor is cylindrical in shape with the stator rotatably disposed inside, disposed opposing the surfaces of the stator magnetic pole teeth through an air gap which is of a uniform dimension throughout the circumference between the surfaces of the stator magnetic pole teeth of the stator and the rotor, and the surface magnetic flux distribution thereof has a substantially sinusoidal wave form circumferentially; a driving means to impress a driving signal in a three-phase single-two-phase excitation mode to three excitation feeding terminals and to control rotation of the stepping motor by a signal from a leakage flux detector for detecting magnetic flux leaking from a notch provided in a rotor yoke; and a means to repeat the processing to control the rotation a predetermined number of times and to issue a warning when normal rotation is not obtained. As a result, the device can detect stopping of the stepping motor due to a steep load change, thereby preventing the motor from being damaged.
A method of driving a stepping motor device according to a fourth aspect of the present invention, the stepping motor device comprising: a stepping motor including a permanent magnet type rotor with a plurality of poles secured to a rotating shaft and a stator having stator magnetic poles with stator magnetic pole teeth in which excitation windings are wound around a plurality of magnetic poles in a star or delta connection, wherein the rotor is magnetized in different directions alternately circumferentially to satisfy the following equation: M=4F/3 where M is the number of poles of the rotor and F is the number of the stator magnetic poles, the rotor is cylindrical in shape with the stator rotatably disposed inside, disposed opposing the surfaces of the stator magnetic pole teeth through an air gap which is of a uniform dimension throughout the circumference between the surfaces of the stator magnetic pole teeth of the stator and the rotor, and the surface magnetic flux distribution thereof has a substantially sinusoidal wave form circumferentially; and a driving means to impress a driving signal in a three-phase single-two-phase excitation mode to three excitation feeding terminals and to control rotation of the stepping motor by a signal from a leakage flux detector which detects magnetic flux leaking from a notch provided in a rotor yoke, comprising the steps of: driving the stepping motor by impressing the driving signal in the three-phase single-two-phase excitation mode to the three excitation feeding terminals in a star or delta connection wound on a plurality of magnetic poles of the stepping motor, detecting the signal from the leakage flux detector and comparing the changing speed of the signal of the leakage flux detector with the driving signal of the stepping motor, stopping supply of the driving signal of the stepping motor when there is a difference equal to or greater than a certain value in the comparison results, supplying the driving signals again after a predetermined time, repeating stopping and supplying processes of the driving signal for a predetermined number of times; and issuing a warning when normal rotation is not obtained. Accordingly, reliability of the device for measuring the inter-vehicle distance, direction and relative velocity of a preceding vehicle can be improved.